The ubiquitin/proteasome system is the major mechanism for dynamically regulating selective, time controlled destruction of key signaling molecules. Through the control it exerts on cell cycle progression, transcription, DNA repair, and apoptosis, it critically determines the cellular responses to stress. Importantly, the rate of degradation of substrates by the proteasome varies with physiological and pathological conditions. (We have made the novel observation that radiation or heat treatment directly affects proteasome activity. This has profound implications for radiation- and heat-induced molecular responses. For example, the rate of degradation of IkappaBalpha is slowed, with consequences for NF-kB activation and cell survival. Proteasome structures within cells come in several distinct forms that vary between cells and tissues. This study will use a number of genetic and biochemical approaches to determine the type of proteasome and the subunits that are most affected by heat and/or radiation. The hypothesis that hsp90, a known endogenous inhibitor of proteasome function, mediates heat- and radiation-induced proteasome blockade will be tested. Proteasome structure will be linked to biological function using genetically deficient cells and the biological consequences of proteasome diversity for the response to heat and/or radiation will be evaluated in vitro and in vivo.) Many studies have focused on transcriptional activation of genes following heat and radiation. Here, we will evaluate the importance of a major non-transcriptional control mechanism that can rapidly, selectively, and simultaneously co-ordinate multiple facets of the molecular response of cells to therapeutic agents. We believe that this cellular control mechanism is critical in determining cellular responses to heat and radiation and serves as a potential novel target for therapeutic intervention.